Substantial efforts have been made in recent years to increase the power conversion efficiency of both silicon and III-V compound solar cells. These efforts have established that gallium arsenide has potentially about a 4% power conversion efficiency advantage over silicon and further that this advantage is even greater when III-V ternary compounds are used together with gallium arsenide to form the solar cell. Of particular interest in this latter class of structures is a cell which includes an N-type gallium arsenide substrate upon which a P-type layer of gallium aluminum arsenide, Ga.sub.1-x Al.sub.x As, is epitaxially grown to form a P-type region in the substrate and defining a PN junction therein. One structure of this general type is shown, for example, in an article entitled "From Photons to Kilowatts, Can Solar Technology Deliver?" by Joel duBow, Electronics, Vol. 49, No. 23, Nov. 11, 1976, at page 89.
Cells such as those shown in the above duBow article have been fabricated using infinite melt liquid phase epitaxial (LPE) techniques such as those disclosed, for example, in U.S. Pat. No. 3,994,755 of G. S. Kamath et al, issued Nov. 30, 1976, and in allowed U.S. application Ser. No. 717,806 of G. S. Kamath et al, entitled "Method for Growing Thin Semiconducting Epitaxial Layers", filed Aug. 23, 1976, both assigned to the present assignee. The method of this latter copending application is referred to as the vertical graphite slide bar technique for epitaxial growth and utilizes an enclosable vertical graphite slide bar holder for supporting gallium arsenide substrates prior to and during epitaxial deposition. This holder serves to establish thermal and saturation equilibrium between the substrate and the infinite melt solution in which GaAs and Ga.sub.1-x Al.sub.x As epitaxial layers may be formed on GaAs substrates. All of the above-identified disclosures are incorporated fully herein by reference.
In order to minimize the resistivity of the P and N-type regions of the semiconductor structure and thus minimize the series resistance of regions of the above-described gallium aluminum arsenide gallium arsenide solar cells, P-type dopants such as zinc and germanium have previously been introduced into the P-type layers of the solar cell structure during epitaxial growth from solution. One such Zn doped solar cell is disclosed, for example, by H. J. Hovel et al in an article entitled "Ga.sub.1-x Al.sub.x As - GaAs PPN Heterojunction Solar Cells", Journal of the Electrochemical Society, September 1973, pp. 1246-1252, incorporated herein by reference. These dopants serve to increase the carrier concentration and thus lower the bulk resistivity of the structure on the P side of the PN junction therein. While these particular P-type dopants, i.e., zinc and germanium, have proven generally satisfactory in fabrication of certain types of solar cells, both of these dopants also create certain disadvantages which have been eliminated by the present invention. That is, the element zinc has a high vapor pressure and an anomalous behavior as a diffusant in GaAs and as such poses control and stability problems in device processing. On the other hand, germanium has proven unsuitable for use as a P-type dopant with high aluminum concentrations in a Ga.sub.1-x Al.sub.x As P-type layer. That is, when x is greater than about 0.85 in Ga.sub.1-x Al.sub.x As, germanium becomes a deep level impurity in the GaAs bandgap, thus reducing the number of ionized carriers available in the semiconductor and rendering it ineffective as a means of lowering the resistivity on the P side of the PN-junction of the solar cell structure. The result, of course, is an increase in forward resistance in the cell, and this causes heating in the cell and results in power losses during solar cell operation. Additionally, for values of x greater than 0.85 in Ga.sub.1-x Al.sub.x As, the carrier concentration due to germanium is less than 10.sup.16 atoms per cubic centimeter range at maximum germanium solubility.